avoid generating iv name each time in accessors

mrblib/class.rb

@@ -2,14 +2,16 @@ class Module
   # 15.2.2.4.13
   def attr_reader(*names)
     names.each{|name|
-      define_method(name){self.instance_variable_get('@'+name.to_s)}
+      name2 = ('@'+name.to_s).intern
+      define_method(name){self.instance_variable_get(name2)}
     }
   end
   # 15.2.2.4.14
   def attr_writer(*names)
     names.each{|name|
-      aset = (name.to_s+"=").intern
-      define_method(aset){|v|self.instance_variable_set('@'+name.to_s,v)}
+      name2 = ('@'+name.to_s).intern
+      name = (name.to_s+"=").intern
+      define_method(name){|v|self.instance_variable_set(name2,v)}
     }
   end
   # 15.2.2.4.12